Hi,
At work, i used a Soekris Appliance, the first model(2001) : cpu 133 MHz with 64 Mo. I use it for PF and isakmpd vpn. I have also a NAS(3To) connected in our network. Clients(there are 3) send his backup at work using our vpn, and tranfert their datas to our nas (through an ipsec tunnel). Since, we put this firewall, we have some errors on copy.

I use top command on the OpenBSD firewall, and have this :
idle : 81 % ; Free MEM : 24 M

I tried to verify system with : systat and swapctl, all seems to me good. But

Do you think, that this appliance is too slow for what i do ? (transfert FTP through an ipsec tunnel).

What kind of copy errors? Have you considered using or testing with rsync?

Did you test data transfers through this box before you dropped it into your operational environment?

In this case of old hardware, it is a good idea to run hardware tests (like memtest), and suspect the power supply (i.e. you may find that replacing the power brick-power board or power supply fixes your problem).

You also may find the NIC hardware is browning out. Where did you get this 10 year old box from? Was it working correctly when they stopped using it and did they store it correctly after they stopped using it?

What kind of bandwidth do your VPN tunnels pump these backups at? Are these VPN tunnels competing with each other? Is there errors with only 1 VPN tunnel?

Also, what release are you running on that box? OpenBSD 4.0 only gets tested on so many hardware boxes.